Nuclear Chemistry Chemistry IH – Chapter 25 Chemistry I – Chapter 21

Radioactivity Much of the early important research about radioactivity was completed by Marie Curie ( ). Much of the early important research about radioactivity was completed by Marie Curie ( ). She discovered radioactive decay, the spontaneous disintegration of some elements into smaller pieces. She discovered radioactive decay, the spontaneous disintegration of some elements into smaller pieces.

Nuclear Reactions vs. Normal Chemical Changes Nuclear reactions involve the nucleus Nuclear reactions involve the nucleus The nucleus opens, and protons and neutrons are rearranged The nucleus opens, and protons and neutrons are rearranged opening the nucleus releases a lot of energy that holds the nucleus together – called binding energy opening the nucleus releases a lot of energy that holds the nucleus together – called binding energy “Normal” Chemical Reactions DON’T involve the nucleus. They only involve electrons. “Normal” Chemical Reactions DON’T involve the nucleus. They only involve electrons.

Mass Defect Some of the mass can be converted into energy Some of the mass can be converted into energy Shown by a very famous equation! Shown by a very famous equation! E=mc 2 EnergyMass Speed of light

Why decay occurs Nuclei of radioactive isotopes are unstable. There are various types of instability and radioactive decay

Types of Radiation Alpha (ά) – a positively charged helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons Alpha (ά) – a positively charged helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons Beta (β) – an electronBeta (β) – an electron Gamma (γ) – pure energy; called a ray rather than a particleGamma (γ) – pure energy; called a ray rather than a particle

Other Nuclear Particles (FYI, Don’t copy) Neutron Neutron Positron – a positive electron Positron – a positive electron Proton – usually referred to as hydrogen-1Proton – usually referred to as hydrogen-1 Any other elemental isotopeAny other elemental isotope

Penetrating Ability Write down what is needed to stop each type of radiation

Balancing Nuclear Reactions The left side (reactants side) of equation and the right side (products side) of equation MUST BE BALANCED Atomic numbers must balance and Mass numbers must balance HOW TO BALANCE: Use a particle or isotope to fill in the missing protons and neutrons. (See next slide.)

Nuclear Reactions Balancing an Alpha emission equation Balancing an Alpha emission equation Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2. Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

Nuclear Reactions Balancing a Beta emission equation Balancing a Beta emission equation Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

Learning Check What radioactive isotope is produced in the following bombardment of boron? 10 B + 4 He ? + 1 n 5 2 0

Write Nuclear Equations! Write the nuclear equation for the beta emitter Co-60.

Artificial Nuclear Reactions New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4 He and 11 B. Reactions using neutrons are called  reactions because a  ray is usually emitted. Radioisotopes used in medicine are often made by  reactions.

Artificial Nuclear Reactions Example of a  reaction is production of radioactive 31 P for use in studies of P uptake in the body P n ---> P + 

Transuranium Elements COPY: Elements beyond 92 (transuranium) made starting with a  reaction DON’T NEED TO COPY: U n ---> U +  U ---> Np  Np ---> Pu  Np ---> Pu 

Nuclear Fission

Fission is the splitting of atoms These are usually very large, so that they are not as stable Fission chain has two general steps: 1. Initiation. Reaction of a single atom starts the chain (e.g., 235 U + neutron) 2. Propagation. 236 U fission releases neutrons that initiate other fissions

Representation of a fission process.

Nuclear Fission & POWER FYI: DON’T COPY Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. 17% of the world’s energy comes from nuclear. 17% of the world’s energy comes from nuclear.

Figure 19.6: Diagram of a nuclear power plant.

Nuclear Fusion Fusion small nuclei combine 2 H + 3 H 4 He + 1 n Occurs in the sun and other stars Energy

Nuclear Fusion Fusion Excessive heat can not be contained Attempts at “cold” fusion have FAILED. “Hot” fusion is difficult to contain

Half-Life HALF-LIFE is the time that it takes for 1/2 a sample to decompose. HALF-LIFE is the time that it takes for 1/2 a sample to decompose. Symbol for half-life is t 1/2 Symbol for half-life is t 1/2

Half-Life Decay of 20.0 mg of 15 O. What remains after 3 half-lives? After 5 half-lives?

Kinetics of Radioactive Decay For each duration (half-life), one half of the substance decomposes. For example: Ra-234 has a half-life of 3.6 days If you start with 50 grams of Ra-234 After 3.6 days > 25 grams After 7.2 days > 12.5 grams After 10.8 days > 6.25 grams

Learning Check! The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours? ANSWER: 8 mg of I-123 remain ANALYSIS: 39/13 = 3 half-lives 39/13 = 3 half-lives After 1 half-life = 32 mg remain After 1 half-life = 32 mg remain After 2 half-lives = 16 mg remain After 2 half-lives = 16 mg remain After 3 half-lives = 8 mg remain After 3 half-lives = 8 mg remain SUMMARY: 64g I-123 x ½ x ½ x ½ = 8 g I-123

Effects of Radiation FYI: DON’T NEED TO COPY

Geiger Counter Used to detect radioactive substances

FYI: DON’T NEED TO COPY

Radiocarbon Dating DON’T COPY: Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation 14 N + 1 o n ---> 14 C + 1 H The C-14 is oxidized to CO 2, which circulates through the biosphere. When a plant dies, the C-14 is not replenished. COPY: But the C-14 in once-living things continues to decay with t 1/2 = 5730 years. Activity of a sample can be used to date the sample.

Sample Problem 25.1 p 806 Carbon-14 emits beta radiation & decays with a t 1/2 =5730 years. If you start with a mass of 2.00 x g of carbon-14 a.How long is three half-lives? b.How many g of the isotope remain at the end of three half-lives?

Answer a. t 1/2 = 5730 years x 3 half-lives = 17,190years half-life b x g x ½ x ½ x ½ = 2.5 x g

Nuclear Medicine: Imaging Thyroid imaging using Tc-99m

Food Irradiation FYI: DON’T COPY Food can be irradiated with  rays from 60 Co or 137 Cs.Food can be irradiated with  rays from 60 Co or 137 Cs. Irradiated milk has a shelf life of 3 mo. without refrigeration.Irradiated milk has a shelf life of 3 mo. without refrigeration. USDA has approved irradiation of meats and eggs.USDA has approved irradiation of meats and eggs.